In magneto-optic recording, data is represented as a magnetized domain on a magnetizable recording medium. Each domain is a stable magnetizable region representative of a data bit. Data is written to the medium by applying a focused beam of high intensity light in the presence of a magnetic field. The recording medium typically includes a substrate, a magneto-optic recording layer, a reflective layer, and two or more dielectric layers that together form the MO stack. In substrate-incident recording, the beam passes through the substrate before it reaches the recording layer. The reflective layer in a substrate-incident recording medium is formed on a side of the recording layer opposite the substrate. The reflective layer reflects the beam back to the recording layer, increasing overall exposure and absorption. In near-field, air-incident recording, the beam does not pass through the substrate. Instead, a solid immersion lens (SIL) is used to transmit the beam across an extremely thin air gap and through the top of the recording medium to the recording layer. The SIL transmits the beam by evanescent coupling across the air gap. In an air-incident recording medium, the reflective layer is formed adjacent the substrate and the thin air gap forms one of the layers in the MO stack from an optical performance standpoint. The recording beam heats a localized area of the recording medium above its Curie temperature. The area is allowed to cool in the presence of a magnetic field. The magnetic field overcomes the demagnetizing field of the perpendicular anisotropy recording medium, causing the localized area to acquire a particular magnetization. The direction of the magnetic field and the resulting magnetization determine the data represented at the domain. With beam modulation recording techniques, the magnetic field is maintained in a given direction for period of time as the beam is selectively modulated across the recording medium to achieve desired magnetization. According to magnetic field modulation (MFM) recording techniques, the beam is continuously scanned across the recording medium while the magnetic field is selectively modulated to achieve desired magnetization. Alternatively, the beam can be pulsed at a high frequency in coordination with modulation of the magnetic field. Examples of various MFM recording techniques are described in The Physical Principles of Magneto-optical Recording, by Masud Mansuripur, Cambridge University Press 1995.
To read the recorded data, the drive applies a lower intensity plane-polarized beam to the recording medium. Upon transmission through and/or reflection from the recording medium, the plane-polarized beam experiences a Kerr rotation in polarization. The Kerr angle of rotation varies as a function of the magnetization of the localized area. An optical detector translates the Kerr rotation angle into an appropriate bit value.